dehumidification in industrial and building applications.pdf

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PDHonline Course M227 (4 PDH)

Dehumidification in Industrial and

Building Applications

2012

Instructor: A. Bhatia, B.E.

PDH Online | PDH Center

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Fairfax, VA 22030-6658

Phone & Fax: 703-988-0088

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Dehumidification in Industrial and Building Applications

A. Bhat ia, B.E.

Course Content

Introduction

Human comfort in conditioned spaces requires relative humidity in middle percentages, on the order of 30to 60 percent. High relative humidity causes human discomfort due to perspiration and inability of the bodyto cool by evaporation of sweat while low relative humidity causes human discomfort due to nasal drynessand problems with static electricity generation. Dehumidification is required to control indoor relativehumidity during warm and moist weather and humidification is required during cold and dry weather.

WHY DEHUMIDIFICATION?

There are many reasons to control humidity in indoor spaces:

1) Meeting the requirement of space usage

2) Providing the healthy and comfortable environment for occupants

3) Preserving the structural integrity of the building

4) Preserving furnishings, wall paper, flooring, paneling and valuable artifacts

5) Preventing rusting of pipes, metal items such as tools, window frames and other ferrous surfaces

6) Providing the specific environment for specific processes

7) Eliminating odors and foul smells

Dehumidification solves four common problems typically encountered in industrial applications:

Moisture Regains (clogging and sticking)

Virtually every substance has some affinity for moisture. Even plastic resins like nylon can collect six to tenpercent of their dry weight in water vapor. In many cases, this presents no problem. In others, moistureregain can affect critical dimensions or make products stick together. The typical home salt shakerillustrates this point when moisture regain on humid days clogs the shaker holes.




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Protection against Condense

When cold surfaces are surrounded by moist air, water vapor will condense on the surface like “sweat”. Thiscan lead to a surprising number of problems. For instance, consumers in a supermarket may not be able tosee frozen foods in a refrigerated display case with a glass door. That may mean thousands of dollars inlost revenue. In other cases, the condensate dripping from the pipe or duct surfaces can spoil the ceiling,flooring, wall paper or discolor the furnishings. In all these cases, the solution is to lower the dew point ofthe air to at value inferior to the temperature of the coldest surface and the only possibility of lowering thedewpoint is by means of dehumidification, there are no other alternatives.

A surface will not have condensation or ice formation on it, if the air in contact with it has a dewpoint lowerthan the surface temperature.

Protection against Corrosion

The water in the air at a high relative humidity is sufficient for a corrosion process to take place. The limitlies at 45-50 %RH: if the humidity lies under that value, there is little risk of corrosion; if the atmospherichumidity lies above 60% the corrosion process will set in. From 60 to 100 %RH the corrosion process willbe importantly accelerated.




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Process/Product Drying- Drying heat-sensitive products

The quality of dry drugs, dry food, hard candy, sugar, chewing gums, capsules, pills and other hygroscopicmaterials can only be maintained, if it is kept in contact with air at a low relative humidity. Sensitive productscan begin to deteriorate at just 30% relative humidity (RH). Many products require a constant RH of 10-15%or less for stability and many types of products must be dried to low moisture levels but cannot standexcessive heat. Enzymes, for instance, are destroyed by heat, and if yeast is dried with very hot air, itcannot work properly.

How to Remove Moisture

There are several methods of drying air:

1) Make-up air method - This method uses the principle of dilution, removing a portion of the moistureladen air from a space and replacing it with drier air. This method works well in parts of the countrywhere outside air is drier. This method is difficult to apply in summer months and expensive to operatein winter due to heating costs.

2) Compression of air – As air is compressed beyond saturation, water vapor in the air begins tocondense, and leaving air is drier. When air is compressed, the dew point is raised, that is, thetemperature at which water vapor will condense is raised. Using compression to dry air is effectivewhen small quantities are needed. This method has high installation and operational costs and is mostcommon when less than 100cfm of dry air is required.

3) Heating or Evaporation - If the room (and thereby the air) is heated, the air will be able to hold morewater, and the relative humidity will decrease. However, heating asks for a well-insulated building inorder to meet the demands of building rules and regulations and in order to keep down the cost ofenergy. Note that approximately 1052 Btu are needed to evaporate one pound of water.

4) Dehumidification

! Refrigeration dehumidification – As air is cooled, relative humidity increases; water beginscondensing when the air is cooled below its dew point or saturation temperature. Air conditioners

and mechanical dehumidifiers are both refrigeration dehumidifiers. This method is effective fordesired conditions down to 45 percent RH for standard applications and has moderate capital andoperating costs.

! Desiccant dehumidification - Desiccant dehumidifiers use special materials that absorb or holdmoisture. The material is unique in that it does not change its size or shape when acquiring themoisture and can be regenerated by applying heat. This technique is used effectively to dry air inthe range of 0 to 50%RH. It has a relatively expensive capital expense as well as a high operationalcost. Desiccant dehumidifiers are effective in controlling humidity at low temperatures, and havebeen widely applied in powder processing and product handling activities such as granulated sugarstorage silos, packaging rooms, ammonium nitrate fertilizer storage buildings, and candy wrapping.

If you use too little dehumidification, materials dry too slowly, increasing the chances of secondary damage;

if you use too much, you may waste your client's money. If the application is drying of products, it is usuallybest to first get rid of excess water followed by evaporation and dehumidification. Specifically for comfortand critical low temperature manufacturing applications, you have two possibilities: Refrigeration andDesiccant Dehumidification. A further discussion on “how to produce dry air” will follow later in the course.




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SECTION – 2 PSYCHROMETRICS

The rudimentary basis for understanding any air conditioning, moisture control and dehumidification ispsychrometrics, the study of the properties of air-water mixtures. Psychrometrics is an essential foundationfor understanding how to change air from one condition to another. But before we discuss this further, let usfamiliarize ourselves with some of the often used terminologies and their definitions.

1) Dry Bulb Temperature (DBT) - The Dry Bulb Temperature refers to the ambient air temperaturemeasured using a normal thermometer freely exposed to the air but shielded from radiation andmoisture. It is called "Dry Bulb" because the air temperature is indicated by a thermometer not affectedby the moisture of the air.

The dry bulb temperature is an indicator of heat content of the air. As the DB temperature increases,the capacity of moisture the airspace will hold also increases. This reduces RH and allows for drying.The dry bulb temperature is usually given in degrees Celsius (ºC) or degrees Fahrenheit (

ºF). The SI

unit is Kelvin (K). Zero Kelvin equals to -273oC.

2) Wet Bulb Temperature (WBT) - The Wet Bulb temperature is the temperature measured by using athermometer whose glass bulb is covered by a wet wick/cloth.

The wet bulb temperature is indicator of moisture content of air. Wet bulb temperature is very useful inindustrial drying processes as the difference between the dry bulb and wet bulb temperature is ameasure of the drying capacity of the air. At 100% relative humidity, the wet bulb temperature equalsdry bulb temperature.

3) Humidity- Humidity is the quantity of water vapor present in air. It can be expressed as an absolute,specific or a relative value.

! Absolute humidity is the actual mass of water vapor present in the air water vapor mixture. Absolute

humidity may be expressed in pounds of water vapor (lbs).

! Specific humidity or (humidity ratio) is the ratio between the actual mass of water vapor present inmoist air - to the mass of the dry air. The humidity ratio is very useful in dehumidification because atnormal pressures, this ration represents an actual measure of water mass relative to air mass.Therefore, the humidity ratio does not change with changes in air temperature. Specific Humidity isnormally expressed in grains of water vapor /lb of dry air and may also be expressed in the units ofpounds of water vapor/lb of dry air or grams of water vapor /kg of dry air.

! Relative Humidity or RH is the actual amount of moisture in the air compared to the total ormaximum moisture the air can hold at a given temperature. When air has 50 percent relativehumidity (RH), we say it is 50 percent saturated (the terms are numerically so close that we usethem interchangeably). Obviously, as air approaches 100 percent saturation, it can take on less andless water until at 100 percent RH, the air cannot hold more water.

Relative humidity is determined by comparing the "wet-bulb" and "dry-bulb" temperature readings.Dry bulb and wet bulb temperatures are taken simultaneously and then plotted on a psychrometricchart. Relative humidity is determined by the value at the intersection of two temperature lines.




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4) Dew Point Temperature – The Dew Point is the temperature at which water vapor starts to condenseout of the air and becomes completely saturated. Above this temperature the moisture will stay in theair. When we find dew on grass in the morning, we know that the temperature of grass reached the dewpoint sometime during the night before. As air cools during the night, it can hold less and less moistureand the relative humidity will get very high. Finally, air becomes so cool and saturated; it releases somemoisture as dew.

The dew point temperature is an indicator of the actual amount of moisture in air. The local dew pointtemperature is sometimes given on weather reports and when the dew point temperature is notprovided, the nighttime low temperature is a good estimate of the dew point temperature and theamount of moisture held in the air.

The dew-point temperature is expressed in degrees and like humidity ratio; it represents an absolute

measure of the moisture in the air at a constant pressure. If the dew-point temperature is close to theair temperature, the relative humidity is high, and if the dew point is well below the air temperature, therelative humidity is low.

The Dew Point temperature can be measured by filling a metal can with water and ice cubes. Stir by athermometer and watch the outside of the can. When the vapor in the air starts to condensate on theoutside of the can, the temperature on the thermometer is pretty close to the dew point of the actual air.

5) Grains of Moisture: Term used to express the weight of moisture per pound of air (14 cubic feet). 7000grains is the most that can be held in one pound of air. Since water weighs 8.34 pounds per US gallonand since there are 8 pints in one gallon, 7000 grains is equal to about 1 pint of water. Air cannot holdmuch moisture as far as poundage of water. In indoor comfort applications, the “space-neutral”conditions are defined as 75ºF, 50%RH, which corresponds to 65 grains per lb of dry air.

6) Vapor Pressure – This is the pressure exerted by the water molecules in air. Sometimes expressed inpsia or in-Hg, it is very useful in sorption dehumidification because water vapor moves through air toequalize differences in vapor pressure. The operating principle of desiccant dehumidifiers is to movewater vapor through difference in vapor pressure.




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7) Psychrometric Chart – The psychrometric chart shows the relationship between the air volume,temperature and relative humidity and is used to calculate specific humidity, dew point (wet bulbtemperature) and vapor pressure.

! On the psychrometric chart, the dry-bulb temperature is shown along the bottom axis and theconstant dry bulb temperatures appear as vertical lines in the chart.

! Lines of constant wet bulb temperatures run diagonally from the upper left to the lower right in thepsychrometric chart.

! Combining the dry bulb and wet bulb temperature in a psychrometric chart, gives the state of thehumid air.

! The vertical axis represents humidity ratio, the moisture content of the air mixture, expressed in thischart as grains of moisture per pound of dry air. Some charts show humidity ratio as pounds ofmoisture per pound of dry air, where 7000 grains = 1 pound.

The laws of physics dictate that the air will hold a specific amount of water (measured in lbs or grains)based on the dry bulb temperature of the air. As air temperature decreases, the amount of water that canremain in the air as vapor also decreases. This means that if the temperature falls and the amount of watervapor in the air remains the same, the vapor must become liquid (visible droplets). The air at this point is“saturated” with water vapor. If the maximum humidity ratio for each dry bulb temperature is plotted, the

curve connecting these points is called the Saturation Line or the 100% Relative Humidity Line. Anyadditional moisture in the air mixture would be visible droplets or fog.




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Psychrometric Processes

There are broadly four types of energy changes:

1) Sensible heat occurs when heat is added without the addition or reduction of moisture.

2) Sensible cooling is the reverse of above

3) Latent heat also known as humidifying, is the addition of moisture without changing the dry bulbtemperature.

4) Latent cooling or dehumidification is the removal of moisture.

Fig# (a) below shows how these processes are displayed on the psychrometric chart. Rarely will theseoccur as shown but will be rather be a mixture of them. Fig# (b) shows dehumidification by cooling is acombination of sensible and latent cooling and sensible heating. First the system cools the air to reduce thedry bulb temperature to the dew point, then the latent cooling reduces the absolute humidity and finally theair is reheated increasing its dry bulb temperature before distributed to the space.

Fig# (b), Psychrometric Process

hT = Total Cooling, A-B

hS = Sensible Cooling, C-B

hL = Latent Cooling, A-C

WA = Specific humidity A (room air)

WB = Specific humidity C (supply air)

!T = Total temperature rise, B-D

!GR = WA - WB

Note an important aspect of dehumidification application:

Air is compressible fluid and its volume is represented by the following equation:

V = K (T/P)

Where




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V = Volume

T = Temperature

P = Pressure

K = Constant

As the air temperature increases, its total volume increases and decreases on reduction of temperature.Pressure has the opposite effect; as pressure increases volume decreases. Water (moisture), however isnot compressible. Therefore given a specific amount, it will always occupy the same amount of volume. Thepsychrometric chart illustrates this concept. As moisture laden air is heated or cooled, the air volumechanges but the moisture does not. Thus there is a change in relative humidity, without a change in actualwater content. This is important to understand because water damage occurs at an absolute humidityconcentration regardless of its relative humidity. A further description is noted below:

What is a good measure for moisture control - Relative Humidity, Dew Point or Absolute Humidity

Relative humidity has been one of the most inexpensive means of humidity measurement and control and iswidely used and understood, but there is fair amount of confusion on this point as well.

Relative Humidity - Relative Humidity measurements take into consideration both the sensible and latentcomponent of air. For example relative humidity reflects the relative amount of water that air can hold at a

given temperature. Change the temperature and the humidity will change even if net amount of moisture inthe air remains the same. In fact a 1ºF change in room temperature can change the relative humidity by2%. If outside air at 85ºF and 60% RH is cooled to 72 degrees without any moisture removal, the RH willincrease to almost 90%. This temperature effect on relative humidity makes it useless as a moisture controlparameter.

The mold growth is closely linked to relative humidity on the surface of the material. This material may be inthe ductwork, in carpet, or on the inside of walls and at a very different temperature than what we wouldtypically measure in the space. The space temperature and associated RH measured will likely be verydifferent than on the surfaces of concern. So while RH on the surface of a material is a good indicator ofmold growth, RH in the space cannot be used to predict or control the moisture content on mold vulnerablesurfaces.

The key to controlling moisture in buildings is to directly measure the latent content (moisture) of air

separate from its sensible (dry bulb) component. This is where measurements such as dew point andgrains/lb of dry air come into play. Temperature will not affect either of these measurements.

Dew Point -Dew point is really a predictive measure that indicates the temperature at which moisture in theair will reach 100% and condense onto a surface. To ensure high moisture levels or condensation does notoccur on those surfaces, dew point levels in the air should be controlled in the building to belowtemperature of the coldest surfaces in a space (e.g. slab on grade floor or un-insulated duct sections). Thiswill ensure that moisture conditions never approach the point that would sustain mold growth on coolersurfaces. . Humidity Ratio (Grains)

Absolute Humidity -The measure of Grains/lb of dry air called the humidity ratio and is an indication of themass of water vapor in air. This measure is most useful to designers and operators of dehumidificationequipment because it can be used to predict the specific amount of energy necessary to remove a givenamount of moisture from air. This measure is ideal for building control, where your control strategies arebased on maintaining a given target level of moisture control.

Figure below shows the relationship between dew point, grains and relative humidity. This chart shows howRH can change with temperature but the latent content of air is unaffected by temperature. For a designer achart like this could be used to select the ideal conditions desired in the space for comfort in terms oftemperature and relative humidity. The control strategy for the space would then be to maintain the latentconditions in the space that are equivalent to that design target.




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SECTION -3 REFRIGERATION & DESICCANT DEHUMIDIFICATION

REFRIGERATION DEHUMIDIFICATION

Principle: Lowering air temperature decreases the air's ability to hold moisture.

As air is cooled, relative humidity increases; water begins condensing when the air is cooled below its dewpoint or saturation temperature. The air becomes drier in absolute terms, but is now saturated i.e. itsrelative humidity [RH] is close to 100 %. If a low RH is needed in addition to a low absolute moisture level,the air can be heated after it leaves the cooling coil. This method is effective for desired conditions down to45 percent RH for standard applications.

However cooling air just for drying is usually not practical as this method has significant capital andoperating costs. Normally, this method is reserved for applications where cool air is needed anyhowespecially in drier and humid locations requiring air-conditioning and secondly where high outdoor air needsdrying only slightly lower than the incoming ambient.

Evaporator Optimization

Refrigeration dehumidifiers operate in many modes of operation in order to insure continuous moistureremoval. To achieve this, the system’s design incorporates many different critically sized components thatmust function seamlessly. A cooling coil (evaporator) must be designed to transfer the required energy and




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shall be selected to provide a minimum 52ºF supply air temperature, while still operating at standard chilledwater temperatures. It is often necessary to utilize cooling coils very deep in the direction of air flow withwide fin spacing when circulating anti-freeze fluids at temperatures as low as 32ºF without icing the coilsurface. An important distinction of conventional air conditioner cooling coil v/s refrigeration dehumidifiercooling coil is

1) A conventional air conditioner will generally use three (3) or four (4) rows with 400CFM per ton (12,000

BTU's) to be removed. This combination provides a high sensible-to-latent ratio, which is key to thesystem achieving a high EER (energy efficiency ratio). A dehumidifier will use six (6) to eight (8) rowswith a reduced air volume of 200CFM per ton. This provides a high MRE (moisture removal efficiency)value. This becomes important in high humidity applications such as pool facilities, industrial plants andtreatment of 100% outside air.

2) The second major impact is that the air conditioner type coil creates an approach temperature (thedifference between leaving air temperature across the coil and the actual refrigerant temperature in thecoil) of 12°F to 15°F (6°C to 7°C), while the dehumidifier design creates an 8°F to 10°F (4°C to 5°C)approach. This is important in part-load situations where the leaving air temperature limits are 52°F orlower for the air conditioning design.

Refrigeration Dehumidification Limitations

In drier parts of the country moisture removal by air conditioning is usually sufficient. However, air

conditioners are designed to cool more than they dehumidify air. This can cause a problem in the warm,humid climates of coastal areas such as Florida, where air-conditioning alone can’t keep up with thedehumidification and the relative humidity inside a space can get quit high. Summer dehumidification needsare much greater than the need for cooling air only and therefore many a time air conditioning anddesiccant dehumidification may be both needed almost anytime of the year. As a passive measure, inwarm, humid climates, it is very important to stop unwanted infiltration of air and limit air change byventilation. By repeatedly returning air to the air conditioner, enough moisture can be "wrung out" of air tohelp keep indoor humidity levels below the mildew alert level. Drier air takes less energy to cool so energyis saved, too.

DESICCANTS

Desiccants remove water vapor by chemical attraction caused by “differences in vapor pressure”. When air

is humid, it has a high water vapor pressure. In contrast, the surface of dry desiccant has a very low vaporpressure so the water vapor moves out of the humid air onto the desiccant surface to eliminate the vaporpressure difference. Eventually, the desiccant surface collects enough water vapor to equal the vaporpressure of the humid air. Then, it must be dried (reactivated) by applying heat before it can be used toremove more moisture from the air stream. In general

• Temperature for reactivation: 300 - 350oF (150 -175ºC)

• Heat required for reactivation: 2050 Btu/lb of water removed (4800kJ/kg)

Desiccant Wheel Construction & Operation

The desiccant structure is formed into the shape of a wheel. The wheel constantly rotates through two

separate airstreams. The first air stream, called the process air, is dried by the desiccant. The second airstream, called reactivation or regeneration air dries the desiccant. The desiccant material is contained in thewalls of thin air channels that extend through the depth of the wheel. Channel diameters vary, but typicallyare about 2 mm. This looks like a honeycomb which is open on both ends. Air passes through thehoneycomb passages, giving up moisture to the desiccant contained in the walls of the honeycomb cells.Wheel diameter depends on how much air must pass through it. Large airflow volumes require a largerdiameter wheel.

The figure below illustrates the simple principle of desiccant wheel operation.




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The desiccant cycle follows four steps:

Step#1 is sorption, where the process air passes through a portion of the rotating desiccant wheel, givingoff its water vapor to the desiccant. As the moisture is removed from the air, the desiccant surface vaporpressure and temperature rises as latent heat in the air is converted to sensible heat.

Step#2, the moisture content in the desiccant reaches equilibrium. The surface vapor pressure is equal tothe vapor pressure of the surrounding air. Without a vapor pressure differential, there is no further moisturemovement to the desiccant.

Step#3 is the reactivation of the desiccant, when the surface vapor pressure rises as the desiccant isheated. Because the vapor pressure of moisture in the desiccant is higher than that in the surrounding air,moisture leaves the desiccant. The moisture content is reduced, but vapor pressure remains high becauseof the higher temperature.

Step#4 is the cooling of the desiccant. The vapor pressure at the desiccant’s surface reduces rapidly withcooler temperatures. When the desiccant is cool, the vapor pressure once again is lower than the

surrounding air, so it can collect moisture again.

Because a desiccant dehumidifier removes water vapor rather than condensed liquid from the air, there isno risk of freezing. This type of equipment is most often used for applications requiring dewpoints below50ºF.

Desiccant Key Facts

1) Desiccant wheels are designed to operate with either “a 25% area for reactivation and 75% area forprocess (25/75 split)” or with “50% area for reactivation and 50% for process (50/50 split)”. Generally,the 25/75 split is used for industrial dehumidification, low dewpoint and compact desiccant coolingapplications. The 50/50 split is more often used for commercial cooling applications or applicationwhere low temperature waste heat is available for reactivation.

2) Heat of reactivation energy can be supplied either with electric resistance, (indirect) steam, (indirect)hot water, or direct (or indirect) fired natural (or propane) gas. Additionally, solar or waste heat sourcesmay be utilized. Desiccant wheels are designed for regeneration temperatures up to a maximumtemperature of 350°F. In addition to the regeneration temperature, several other factors influence theperformance of the desiccant wheel. Process and regeneration inlet humidity and temperature,regeneration to process flow ratio, face velocity, and wheel diameter & rotational speed all has animpact on performance.




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3) Energy to drive the desiccant cycle is proportional to the mass that is heated and cooled. The moredesiccant and water that is heated, the more energy is used. The cost of operation depends primarilyon the cost of the reactivation energy plus the cooling energy.

4) Desiccant process is the reverse of evaporative cooling. When water is evaporated into air, the heatneeded for evaporation comes from that same air and the sensible temperature falls. Conversely, whenair is dehumidified, the heat needed to evaporate the water originally is liberated and the temperature of

the air stream rises.

5) With desiccants, moisture removal occurs in the vapor phase. There is no liquid condensate.Consequently, desiccant dehumidification can continue even when the dew point of the air is belowfreezing. This is different from cooling- based dehumidification, in which the moisture freezes and haltsthe process if part of the coil surface is below 32°F.

6) The ideal desiccant would have an infinitely high surface area for collecting moisture, and an infinitelylow mass, since the required heating and cooling energy is directly proportional to the mass of thedesiccant and the mass of the machinery which presents the desiccant to the air-stream. The heavierthe desiccant assembly compared to its capacity, the more energy it will take to change its temperature-which accomplishes dehumidification.

7) One aspect of desiccant wheel behavior can be confusing to the first-time user of the technology; airleaves a desiccant wheel dry, but leaving air is 30 to 40ºF warmer than inlet air. For example, if air

enters a desiccant wheel at 70°F and 50% RH, it will leave the wheel at about 100°F and 4% RH. Thisnon-intuitive behavior becomes easier to understand as the reverse of evaporative cooling. When wateris sprayed into air, it evaporates by using part of the sensible heat in the air—so the dry bulbtemperature falls as water vapor is added to the air. Desiccants produce the opposite phenomenon. Aswater vapor is removed from air, the dry bulb temperature of the air rises. The amount of temperaturerise depends on the amount of water removed. More water removal produces a greater temperaturerise.

The initial user naturally asks: how can desiccant systems save cooling energy, if dehumidification addssensible heat to the air? Part of the answer is that some heat is moved to reactivation by a heatexchanger. The rest of the answer depends on the application. For example, if air is dry, it may not benecessary to cool it if the space is already overcooled—as in a supermarket, where display cases coolthe aisles as well as the product. Alternatively, dry air can be cooled using low-cost indirect evaporativecooling such as cooling towers, or with highly efficient vapor compression systems operating at highevaporator temperatures. In such cases, desiccants can save energy and energy cost.

Desiccants – Adsorption or Absorption

Desiccants do not absorb moisture, they “adsorb”. Absorption occurs when one substance is chemicallyintegrated into another. Adsorption means moisture is held inside a desiccant by physical bonds; nochemical change actually occurs.

1) Adsorption is a physical process in where moisture is held on the surface of the material without anychange of in the physical or chemical structure of the material. Water is adsorbed or held on the surfaceof the material and in the pores, Adsorbents are mostly solids. Typical examples of Adsorbents used fordehumidification are silica gel, molecular sieve and activation alumina.

2) Absorption involves a change in the physical or chemical structure of the material and it is in generalnot easy to reactivate the material. Absorbents are generally liquids or solids; solid sorption materialsinclude sodium chloride salt (NaCl) and liquid sorption systems use calcium chloride solution, glycol orkathene fluids to absorb moisture from an air stream. Water is then boiled out before the sorption liquidis ready to absorb moisture again. Breweries, food processing and natural gas gathering facilities useliquid sorption equipment. Neither of the above systems is generally used for abrasive blastingoperations.

Desiccant Classification




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Desiccants can be either liquids or solids, and there are many different materials of both types. However,the great majority of systems built for commercial and industrial applications use solid adsorbents such assilica gel or activated alumina.

• Silica gel - SiO2 - is a hard, adsorbent, crystalline substance and very porous. Voids are about 50 -70% by volume and adsorb water up to 40% of its own mass. The bulk density of silica gel is 480 -720 kg/m

3. The specific heat capacity is 1.13kJ/kg-K.

• Activated alumina is about 90% aluminum oxide Al2O3 and very porous. Voids are about 50 - 70%by volume and adsorb water up to 60% of its own mass. The bulk density is 800 - 870 kg/m

3. The

specific heat capacity is 1.0 kJ/kgK.

In weighing the various choices of desiccants, it is important to consider the ambient and the spaceenvironment in relation to the desiccant's individual capabilities. Key parameters include: initial humidity;amount of moisture acceptable in space; volume of product; Moisture Vapor Transmission Rate (MVTR);and the maximum level of RH permitted within the space.

COMPARING DESICCANTS TO REFRIGERATION DEHUMIDIFICATIONBoth desiccant dehumidifiers and mechanical refrigeration systems can remove moisture from the air, sothe question is - which type is best suited for a given application? There really are no simple answers to this

question but in general cooling-based dehumidification handles the moisture load occurring at highdewpoints; desiccant-based dehumidification removes the moisture load at lower dewpoints. The mix of thetwo technologies depends on the characteristics of the specific application. The generally acceptedguidelines which most designers follow are listed below:

1) Evaluating the dewpoint control level: When the required moisture control level is comparatively high(above 50ºF dewpoint), cooling-based dehumidification is economical, in terms of both operating costsand initial equipment cost. Below this level, costs increase because precautions need to be taken toprevent condensed water on the cooling coil from freezing.

Although water does not freeze until temperatures fall below 32ºF, a dehumidification system may haveto deliver air below that level to maintain a room below 50ºF dewpoint. Without protective measures, acooling-based dehumidifier providing air at low dewpoints can freeze. Equipment fitted with freeze-protection devices costs more and has higher operating costs/kg of water removed. Under these

circumstances, desiccants become more economical than cooling-based systems at low dewpoints.In general

! If the dew point is above 50 degrees, consultants recommend refrigeration dehumidification.

! If your dew point falls below 40 degrees, desiccant dehumidification provides the best solution.

2) Gauging relative humidity sensitivity: When a process needs low moisture level in absolute terms,but can tolerate a high RH, a cooling-based dehumidification system can be cost effective without theneed for desiccants. For instance in food storage areas, an ideal temperature for fruit and vegetablestorage might be 40ºF. Of course, the dewpoint must be lower than that. If the RH is below 90%, thefruit can dry out and lose value. Because the product needs both low temperature and high humidity,cooling-based systems are ideal. In contrast, other processes might demand a low RH and a lowdewpoint.

3) Determining temperature tolerance: An application with a narrow temperature tolerance needscooling and heating in addition to dehumidification. If the application can tolerate wide temperaturevariations (such as occur in unheated storage, for example), the dehumidification equipment alone maysuffice.

4) Gauging Seasonal Variations: On a seasonal basis, desiccant dehumidifiers can generally be a goodchoice from January through May and September through December, when morning temperatures arebelow 65ºF. Air conditioners may be used from May through September when morning lows are above70ºF. Mechanical refrigerant dehumidifiers offer value from March through November when morninglows are above 55ºF. Both desiccant-based and refrigeration-based dehumidification systems work




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most efficiently when used together. The advantages of each compensate for the limitations of theother.

5) Energy Performance: The amount of energy consumed to remove moisture from inlet air varies bymachine. In general, energy requirements for desiccant dehumidifier are high because of reactivationheat whereas in comparison cooling based dehumidification uses 50% less power. Based upon an inletof 95ºF, 50% relative humidity; desiccant dehumidifiers remove about 1.5 pounds of water vapor per

kilowatt of electricity used. Air conditioners remove 3 pounds and mechanical refrigerant dehumidifiersremove 5 pounds. Desiccant performance is fairly independent of inlet conditions; (1.5 pounds per KW)over a large range of climates whereas air conditioner and mechanical dehumidifier deliver variedperformance based upon inlet conditions.

Summarizing…

Desiccant systems are applicable to existing or new HVAC systems for clean rooms, supermarkets,refrigerated warehouses, ice rinks, schools, restaurants, theaters, hotels, hospital/healthcare facilities, andsituations where one or more of the following situations apply:

1) Low indoor humidity (dew point below 50°F)

2) High latent load fraction (greater than 25%)

3) High outside air fraction (greater than 20%)

4) High electrical cost and low gas costs

5) Available heat source from waste heat, steam, hot water or gas for regeneration of desiccant

6) Can remove up to 50 grains of moisture per lb of air processed

Refrigeration-based dehumidification systems are sometimes more economical than desiccant baseddehumidifiers when higher temperatures and humidity in the conditioned space is acceptable. Mechanicalrefrigeration systems are seldom used for applications below 50% RH at about 22°C or for a dew pointlower than 11°C. Desiccant-based systems are more economical than refrigeration systems at lowertemperatures and lower moisture levels and these keep humidity lower than would be practical withconventional cooling-based systems. Typically, a desiccant dehumidification system is utilized forapplications below 45% RH down to less than 1% RH. Thus, in many applications, a DX or chilled waterpre-cooling coil is mounted directly at the dehumidifier inlet. This design allows for removal of much of the

initial heat and moisture prior to entering the dehumidifier where the moisture is reduced even further.

SECTION -4 SYSTEM DESIGN

Industrial & commercial dehumidification systems are tailored for each project. Therefore, a near-infinitevariety of possible components are available to serve a near-infinite variety of possible applications. Thispresents the need to make many decisions early in the design process, often before cost/benefitimplications are clear. The engineer investigating the system should work closely with potential suppliers tocompare costs and benefits of dehumidification with alternative solutions. Suppliers can be most helpfulwhen key aspects of the project are well defined. The engineer should be able to provide or have on handthe following information.

1) Purpose of the project

The purpose of the project must be clearly understood and documented so that decisions can beproperly ordered according to their importance. If the system must prevent mold growth on starch in astorage silo, a strict tolerance of ±1% RH is not needed. The only concerns are that humidity does notexceed 60% and that condensation does not occur. A simple, inexpensive configuration will do the job.




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Conversely, if dehumidification is needed to prevent the corrosion of lithium, a control with a toleranceof ±5% RH is useless. Above 2% RH, lithium corrodes and gives off hydrogen which eventuallyexplodes. A sensor with a tolerance greater than the critical control level could not start the system intime to prevent the explosion. Understanding the purpose of the project helps prevent bothunnecessary expense and false economy.

2) Establish control levels and tolerances

Next, the humidity and temperature control levels and tolerances needed to achieve the purposes of theproject must be determined. Sometimes, these decisions require research. Other times the relationshipbetween the process and moisture levels is well understood. For example, if a process bogs down inthe summer, but not the rest of the year, humidity tolerance is likely quite wide. The dehumidificationsystem need only remove summer humidity extremes. Or perhaps the supplier of the problem materialcan recommend optimal environmental conditions for processing the product. The control setpoint mustbe established to allow the peak heat and moisture loads to be calculated.

3) Estimate Loads

Without loads, equipment sizes and costs cannot be estimated. For example, a system that holdshumidity at 72ºF and 35% RH is much smaller than one that holds humidity at 72ºF and 25% RH, allother variables being equal. The lower the desired humidity level, the more costly is the system. Highermoisture loads also raise costs. For these reasons, calculating loads is a critical step in system design.Typical moisture loads come from ventilation air, air infiltration, miscellaneous openings, people,products and packaging, and vapor permeation. The following aspects must be carefully examinedwhile calculating the moisture loads:

a) Outdoor Design Conditions: A peak design weather condition is an important element in the loadcalculations. The end user must decide how conservatively the system should be sized. If extremeweather data are used, the system will control humidity throughout the 8760 hr in a typical year.The system will also cost a lot. If some off-hours can be risked, costs may be reduced 20-30%.However, if all moisture loads peak at the same time in extreme weather, the humidity level mayrise above the setpoint. These options are quantified in the Fundamentals Handbook published by

the American Society of Heating, Refrigerating, and Air-Conditioning Engineers according to thepercent of annual hours that weather conditions can be expected to be above certain values. Forexample, the 0.4% values are likely to be exceeded only 35 hr/yr. A less conservative design pointwould be 1% or 2.5% values, which may be exceeded for 70 hr and 219 hr respectively. The enduser must decide which data to use because he is in the best position to assess the economic andsafety consequences of being above specification for short periods.

b) Ventilation air/air filtration: Outdoor air contains large amounts of moisture, especially in summer.Ventilation air is typically introduced through air conditioning and heating units. Fresh air is requiredin most controlled spaces. Most building codes specify a specific amount of air per person (20CFMper person per ASHRAE 62) or per sq ft of occupied space.

The make up ventilation requirements are also influenced by the exhaust requirements. If theexhaust air is more, the space will need more make up air to replace. The problem is especiallyacute in large areas where exhausts may not be obvious. Engineers need to be fully aware of theeffect of insufficient makeup air on humidity-controlled spaces. Higher ventilation rates particularlyin the coastal humid areas will require larger dehumidification equipment. The most cost-effectiveadjustment to building operation is to minimize exhaust air, reducing the cost of dehumidifying themakeup air, however, note that the state or local codes compliance is especially important forhospitals and health care facilities or for any application that exposes humans to toxic fumes.

c) Ventilation Rates for Drying Projects: For moisture control, ventilation rates are dependent uponmoisture loads and the moisture level of supply air. Using ventilation rates to size drying projectscan result in inaccurate amounts of equipment for the job. When exhaust fans are used during aproject, always match dehumidifier air flow with exhaust flow.




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d) Infiltration through Miscellaneous Openings: The moisture-laden air “infiltrates” the buildingthrough opening and closing of doors and through cracks and crevices in a building envelope. Theamount of infiltration varies with the age and integrity of construction and tightness of windows.Opening a door pulls in moist air. Observe and note the number of times a door is opened atbusiest times.

Air locks greatly reduce moist air infiltration. As humidity control levels go lower, air lock doors

become more economically advantageous. With an air lock, it is assumed that equilibrium isreached half way between the inside and outside conditions and that all the air enters the roomeach time the lock opens.

When product must enter or leave a humidity-controlled room on a conveyor, the opening must beconsidered as a possible infiltration source. The infiltration of moist air through large openings suchas ducts can be reduced by supplying a slight overpressure of makeup air to force dry air out ofcracks rather than letting moist air leak in. Sealing building cracks also reduces dehumidificationcosts for a modest investment in caulk.

e) People: People produce moisture and this varies with the level of activity. When workers exhale orperspire, moisture is given off, creating another load source. The rate depends on the exertionlevel. When room loads are calculated, allowances must be made for "visitors" entering and leavingthe room. Doubling "people" estimates to allow for changes in room use is often recommended.

f) Moisture Emission from Products/packaging/appliances: The load from products andpackaging varies greatly by application. In large storage applications, moisture released fromproduct may be the single largest load component. The load equals the difference between aproduct's initial wet weight and the weight when at equilibrium with the lower humidity. In additionshowers, pools, fountains and some commercial processes may also contribute to moisturegenerated from within a building.

g) Vapor permeation: Vapor permeation through building components is typically the smallest loadcomponent. It accounts for less than 2% of the total. This load should receive more attention if thebuilding is very large and moisture permeates across a large surface area or if the control conditionis very low. Below 5% RH, every leak is critical.

SECTION-5 CALCULATING MOISTURE LOADS

To properly apply a dehumidification system, the amount of moisture to be removed must be calculated.

This section provides an approximate yet very practical method for calculating the moisture load in variousapplications. There are many sources of moisture in a facility. A list of the common ones follows:

! Infiltration

! Permeation

! Ventilation and make-up air

! Door and window openings

! People

! Processes

! Product

INFILTRATION & PERMEATION




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Space moisture load is a combination of permeation and infiltration; both of which are often considered thesame thing.

1) Permeation is the migration of water vapor through materials such as brick and wood. Permeation is astraight line function of the difference in interior and exterior vapor pressure (determined by grains/lb).

2) Infiltration is the movement of water vapor through cracks, joints and seals. Infiltration is generallyrepresented in air changes per hour and is not a straight line function because of the two factorsinvolved:

! Each pound of air entering the space will impose a moisture load determined by the difference ininterior and exterior moisture content.

! Since the vapor pressure differs as the moisture content, the vapor will move at a higher velocitythan the air.

The combination of the two factors will be encountered in determining the moisture load. As the differencebetween interior and exterior moisture content increases, there will be a corresponding rate of increase inthe permeation and infiltration loads. Moisture load in a space due to infiltration and permeation is not easilymeasured. Factors such as the actual moisture deviation, materials of construction, vapor barrier and roomsize all have an effect on the vapor migration. Suppliers have been using some basic equations derivedfrom field studies to estimate moisture infiltration and permeation.

Calculating Infiltration and Permeation Load

The combined infiltration and permeation load can be approximated from the following equation:

Lb/HR Moisture = V x GR x F1 x F2 x F3 x F4 / (7000 x C)

Where

V = volume of controlled space in question - ft3

!G = difference between the grains per lb of outside air and the grains per lb desired in the controlled space(Table – I)

F1 = Migration factor due to moisture difference (Table – II)

F2 = Air change factor (Multiplier from Table III)

F3 = Construction factor – Table IV

F4 = Barrier factor (Table V)

C = 13.5 (conversion factor for cu-ft / lb)

7000 = Conversion factor for grains/ lb

Grain Difference (!G)

The !GR (grain/lb) difference between the outside and inside conditions is obtained from the psychrometricchart. For most applications the only information available is the dry bulb and relative humidity or dry bulband wet bulb temperatures. The psychrometric chart is used to plot these two values by finding theirintersection and then following the horizontal line to the right to determine the moisture content in grains perpound.

Alternatively, the table below shows the grains of moisture per pound of dry air at standard atmosphericpressure at relative humidity ranging 10 to 90%. For other values the Psychrometric chart must be utilized.




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TABLE – I

Water Vapor in Air (grains water vapor/lb dry air)

Temperature (ºC / ºF)

-1ºC 4ºC 10ºC 18ºC 22ºC 27ºC 32ºC 38ºC

RelativeHumidity

(%)

30ºF 40ºF 50ºF 60ºF 70ºF 80ºF 90ºF 100ºF

10 3 4 6 8 11 16 21 29

20 5 7 10 16 21 30 42 58

30 7 10 14 22 34 46 65 87

40 9 14 20 30 44 62 85 116

50 12 16 26 39 55 78 108 147

60 14 18 32 48 66 92 128 176

70 17 20 38 54 78 108 158 208

80 19 22 42 62 88 125 173

90 21 24 48 70 100 140 195

The ASHRAE fundamental handbook provides the outside air moisture content for various locations. Thetable below provides some information.

http://www.engineeringtoolbox.com/relative-humidity-air-d_687.html







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Migration Factor (F1)

The rate of infiltration is a function of the magnitude of imbalance between the outside absolute humidityand that inside the conditioned space. The greater the difference, the greater shall be the driving force tomake the vapor pressures equal. The migration factor compensates for this influence. The Table-II belowprovides some industry standard values, which can also be approximated by !G ÷ 35.




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Air Change Factor (F2)

According to ASHRAE, the median number of air changes per hour is 0.5. The actual number of airchanges is influenced by several factors; the most dominate being the size of the room. The larger the roomthe longer it takes to convert one volume. The Table – III below compensates for the reduction in infiltration/permeation on larger or smaller volumes.

Construction Factor; (F3) and Barrier Factor; (F4)




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Another primary factor is the amount of moisture that is allowed to permeate through the walls, floor androof. The construction factor takes into account the effect good vapor barriers and construction materialswill have on the moisture migration.

Table 4 gives factors for common construction materials. This factor will vary between 0.3 and 1.0. Acomposite wall must be modeled and a factor estimated.

Note: If the product of F3 x F4 is less than 0.5, use 0.5. If the room is completely vapor proofed, withcontinuous vapor barrier under the floor (or of all-metal, welded material) the factor may be reduced to 0.3.

Example:

Find the amount of moisture that will permeate the room defined below.

1) Room with 12'' masonry walls

2) Two coats of aluminum paint as vapor barrier

3) Volume of room - 22,000 ft3

4) Outside Design: 90°F db 60% RH ( Read specific humidity from psychrometric chart or alternativelyfrom Table I, which shows 128Gr/lb)

5) Required - To hold in room - 38Gr/lb

Solution:

!G = 128 – 38 = 90

F1 = 2.29 from Table II

F2 = 0.55 from Table III

F3= 1.0 from Table IV

F4 = 0.75 from Table IV

Therefore, amount of vapor able to permeate = 22000 x 90 x 2.29 x 0.55 x 1.0 x 0.75 / 13.5 = 138545Grs/hr

MOISTURE THROUGH INTERMITTENT OPENINGS

Another source of moisture is the opening of doors and windows to the conditioned space or other openingssuch as conveyor passages. In these cases, the amount of moisture is directly proportional to the frequency




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of the opening, the difference in indoor and outdoor moisture content and the wind velocity at the opening.Obviously the first precaution is to assure that openings are adequately vapor-sealed. Then the dryingequipment must deal with the moisture load that comes into a controlled space when the door is open.

The wind velocity will be the most difficult to take into account since it will vary depending on the location ofthe opening with respect to the wind source. Local weather stations can provide details on the normalprevailing direction and speed. However, a guideline is 12 CFM of outside air per square feet of opening.

Assuming that the door is open only for short periods, the amount of moisture can be estimated by thefollowing formula.

Moisture, lb/hr = O x A x !G x 12 / (C x 7000)

Where

O = time in minutes, the area is open per hour

A = area of the door opening in square feet

!G = Difference in specific humidity in grs/lb between controlled space and the ambient. See Table II foroutside conditions and specific humidity

C = 13.5 (conversion factor for cu-ft / lb)

7000 = Conversion factor for grains/ lb

When this equation is used for a fixed opening such as a window, the minute’s open/hr will equal 60 oralternatively if the door opening is specified in number of times each hour the door is opened, the amount ofmoisture can be calculated using equation:

Lb/hr = O x A x !G x F1/ (K x 7000)

Where

O = number of times each hour the door is opened. (If unknown assume personnel door to be opened 2times/hr for every occupant)

A = area of the door opening in square feet


K = 7 (constant)

F1 = Factor from Table I for moisture difference

Example:

Door area - 3' x 7'

Door opens - 6 times each hour

Moisture difference - 90Grs/lb

Solution:

Grains per hour of additional load = 6 x 21 x 90 x 2.29 / (7 x 7000) = 0.53 lbs per hour

MOISTURE ORIGINATING IN THE CONTROLLED SPACE

Moisture or vapor originating in the controlled space comes from any of several sources, depending on theintended use for the space. Three basic sources of moisture originating in space could be remembered as 3P’s:

• Population load, including people and animals

• Product load, brought in by the product




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• Process load

Population Load: People working in an area add moisture to the air because of breathing and theevaporation of perspiration. When animals occupy the controlled space, moisture release is contributed bytheir excrement. How much moisture do people or animals add to a controlled space? This is a function ofthe number of people and their activity: a worker lifting boxes will generate 4 to 8 times the moisture of aworker at a lab bench.

Evaporation Rates of People

Work Type Evaporation Rate (lb/person/hr)

Seated in theater 0.10

Office work, light work 0.20

Medium factory work 0.475

Heavy factory work 0.965

Athletics 1.095

Product Load: Any material manufactured in a controlled area can bring moisture with it and then releasethe moisture into the work area. If the product has an affinity for water, then it may also release the water inthe conditioned room. For example, wet wood brought into a conditioned warehouse will release the waterat a specific rate. This can be determined by measuring the products weight loss over time.

Process Load: The manufacturing process itself may expel moisture into the atmosphere of a controlledspace. Open tanks or cooking vessels will add to the moisture load. Other contributors include open streamexhausts, unvented combustion cycles, and aging or curing cycles.

In the case of, open water tanks, the evaporation rate can be calculated with the following equation.

LB/HR = 0.1 x A x (VPH2O - VP AIR)Where:

A = Surface area of water (square feet).

VPH2O = Vapor pressure of water at water temperature

VP AIR = Vapor pressure of air at its corresponding dew point.

The above equation assumes 10 to 30 FPM air velocity in room.

Vapor pressure table is indicated below and all other values can be obtained from technical publications.




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VENTILATION & MAKEUP AIR

Ventilation air typically introduced through air conditioning and heating units contribute to the moisture load.

This is especially important in the summer months when high humidity is common. The outside air can betakes as 20CFM per person multiplied by expected occupancy in accordance with the requirements of ASHARE 62 guidelines. State or local codes may supersede this figure. Code compliance is especiallyimportant for hospitals and health care facilities or for any application that exposes humans to toxic fumes.Refer to codes and/or the Industrial Ventilation handbook. The formula for calculating moisture load is:

LB/HR Moisture = N x CFM/person x !GR x 60/ (7000 x 13.5)

N = Number of persons

CFM = Volume of outside air introduced (20CFM per person, per ASHRAE 62 guidelines on indoor airquality)


60 = Conversion factor for min/hr

13.5 = Conversion factor for CU.FT./LB

7000 = Conversion factor for GR/LB

SECTION-6 SIZING THE DEHUMIDIFIER

Once the amount of moisture to be removed is calculated, the following equations shall be used to calculateair handling and dehumidifier sizes:

Equation#1

The amount of work done on an air stream to remove moisture:

W = 4.5 x q x (M1 – M2)

Where

W = work in grains (gr) of water vapor/hr, (Gr/hr) or divide by 7000 for Lbs/hr

q = air volume flow (CFM, cubic feet per minute)




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M1 = moisture into the process, gr/lb of dry air

M2 = moisture out of the process, gr/lb of dry air

Equation #2

The amount of energy required to change the temperature of the air (with no change in moisture) iscalculated from equation 2:

The sensible heat in a heating or cooling process of air can be expressed as:

E = 1.08 x q x (T1 – T2)

Where

E = sensible heat for temperature changes only (Btu/hr)


T1= Temperature into the process, (ºF)

T2= Temperature out of the process, (ºF)

Equation#3

The total amount of energy required to change the temperature and humidity of the air is calculated fromequation#3. For refrigeration systems, 1 ton of refrigeration equals 12000 Btu/hr.

Et = 4.5 x q x (h1 – h2)

Where

Et = total energy or energy for temperature and humidity change, (Btu/hr)


h1 = enthalpy into the process (Btu/lb dry air)

h2 = enthalpy out of the process (Btu/lb dry air)

Equation#4

For refrigeration systems, 1 ton of refrigeration equals 12000 Btu/hr, thus refrigeration capacity

ETR = Et / 12000

Where

ETR = Capacity in tons of refrigeration

Et = total energy or energy for temperature and humidity change, (Btu/hr)

SAMPLE CALCULATION

Consider a stream of air must be cooled from 95ºF at 50% RH to 50ºF at 100%RH. Calculate the amount ofrefrigeration capacity needed for the air stream on a per 1000-scfm basis and the amount of moisture to beremoved.

Solution

From psychrometric chart, plot the plot these values by finding their intersection.

Follow the horizontal line to the right to determine the moisture content in grains per pound.

Follow the inclined line to the left to determine the energy contained in the air at each state point (enthalpy)




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The results show:

M1 = 125 gr/lb

h1 = 42.5 Btu/lb

M2 = 53 gr/lb

h2 = 20.3 Btu/lb

Et = 4.5 x 1000 x (42.5 – 20.3)

Et = 99900 Btu/hr

Et = 99900/12000 = 8.3 tons of refrigeration

Amount of moisture removed; W = 4.5 x 1000 x (125 – 53) = 324000gr/hr or 46 lb of water /hr.

In some processes, the air must be dehumidified beyond what a refrigeration system can accomplish. Sayin the example above, if the moisture in the air must be further removed to 7gr/lb, the moisture removed orwork done by the desiccant is calculated by

W = 4.5 x 1000 x (53 – 7) = 207000 gr/hr or 29.6 lb of water /hr.

As a rule of thumb, the heat required for reactivation is approximately 2050 Btu/lb of water removed.Therefore, it would take approximately 29.6 x 2050 = 60680 Btu/hr of energy. Now the volume of airrequired for reactivation can be calculated using equation # 2 above

E = 1.08 x q x (T1 – T2)

Consider heating the reactivation air from the outside air condition of 95ºF to 250ºF, the volume of airrequired to regenerate and dry the desiccant is

q = 60680 / [1.08 x (250 – 95)] = 362 CFM

In the desiccant system, fewer pounds of water are removed, but the moisture is removed in a portion ofpsychrometric chart that refrigeration based technology could not have done effectively because 7gr/lbcorresponds to a 7ºF dew point, which is far below the freezing point of 32ºF. This is why it is important touse a refrigeration system where it is efficient and a desiccant system to dry the air beyond a refrigerationsystem’s capability when necessary.




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SECTION-7 MOISTURE CONTROL IN BUILDINGS

Buildings in hot/warm and humid climates, with high space latent loads, high outside air ventilation rate, lowinterior space humidity requirement or stringent humidity control, will need dehumidification. These types of

buildings include supermarkets, hospitals, labs, clean rooms and theaters.

Understanding the Functions of the HVAC System

In any climate, the HVAC system designer must consider the four basic functions of the HVAC system:

! Comfort control (temperature, relative humidity, dehumidification and air motion)

! Ventilation (proper distribution of makeup air to the interior spaces)

! Contaminant control (filtration)

! Pressurization (makeup and exhaust)

By design, these functions overlap. For example, ventilation helps provide contaminant control andpressurization. In humid climates, two of these functions are essential—providing proper dehumidification (acomponent of comfort control) and positive pressurization—to avoid moisture problems.

TEMPERATURE AND HUMIDITY CONTROL

It is important to understand that there are two components of performance to any air conditioning system.The first component relates to reducing the dry bulb temperature or "sensible energy" of the air. The secondcomponent of performance is the removal of moisture or the "latent energy component" in the air. In air-conditioning, the parameter of performance rating dry bulb/latent energy removal is called the sensible heatratio (SHR). This term identifies that portion of sensible and latent removal that is provided by a particularsystem. Most systems today operate with a sensible heat ratio of 0.8 to 0.95 meaning that the larger portionof energy consumption of the system is devoted to sensible cooling (80 -95%) while the remaining 0.05 to0.5 of the ratio (5-20%) is dedicated to latent heat removal or humidity control.

In modern buildings the need for dehumidification (latent heat removal) has considerably increased whileless dry bulb cooling is needed today than they ever did before. This change in building SHR requirements

is due to the great strides we have made in increasing energy efficiency in buildings. Initiatives like betterwall and roof insulation, better window performance, more energy efficient lighting and appliances, have allreduced internal heat loads and therefore the sensible cooling requirements for buildings. At the same time,those factors that affect the moisture or latent load in buildings such as infiltration, higher outdoor ventilationrequirements and the presence of people, have not changed. The reduced sensible cooling requirement forbuildings has meant that air conditioning systems shall be designed for effective moisture removal and not just on dry bulb temperature control.

DESIGNING FOR PROPER DEHUMIDIFICATION

Dehumidification, or removal of moisture from the supply air, is accomplished in most buildings by coolingthe air below its dew point and condensing moisture from the air. In a typical building, the supply air iscooled to about 52°F as it leaves the air handling unit and is warmed to a temperature of about 75F as it isdiffused into the space. Cooling air to 52°F ensures RH in space to be 50-60%. HVAC systems are typicallydesigned to ensure that they meet indoor comfort conditions at peak cooling loads but seldom does the airconditioning system operate at peak load. At most of the time the air-conditioning system operate at 50-60% loading implying that there is low demand for dry bulb sensible cooling, but the latent load is still at thepeak. How to handle such situations?

Well there are number of ways but the conventional technique is to cool the air to 52°F as it leaves the airhandling unit and provide terminal reheat before it is fed into the space. Reheat is very wasteful. First,excess energy is expended to cool the supply air, then, still more energy is expended to partially re-warmthe same air before it enters the space. During low-load conditions, much more energy may be cancelledout in this manner than actually enters the space to provide cooling or heating.




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These problems can be resolved by careful design, correct air pressurization in buildings, adequatedehumidification capability, proper control and operation of HVAC systems.

Research has also shown that many air conditioner designs, if turned on and off frequently, will actuallyevaporate moisture back into a space before condensed moisture can drain off cooling coils. This on/offcycling can further reduce the moisture removal capabilities of existing equipment. Never oversize therefrigeration equipment; oversized heating and cooling equipment will cycle on and off more frequently and

for shorter periods, leading to poor latent removal performance and frequent temperature swings. Insummer, this is particularly uncomfortable because short cycling prevents adequate dehumidification.

The common practice of setback control, which often means turning cooling equipment off on nights andweekends to conserve energy, is another serious contributor to excessive moisture problems in buildings inhumid climates. When the cooling system is off, infiltration will carry in substantial levels of outside moisturethat get absorbed in building components. This combined with low sensible cooling loads means that thesystem never runs long enough to remove all the moisture in the air and building components. If the drybulb temperature is low enough during operating hours, then high moisture levels and condensation may bewidespread on critical surfaces.

DESIGNING FOR PROPER PRESSURIZATION

Pressurization in buildings is the second, and equally important, issue for producing successful buildings inhumid climates. A building must be positively pressurized relative to the outside; that is, some conditioned

air from inside the building must exfiltrate through the envelope to the outside.

Positive pressurization is normally achieved by providing more outdoor, or makeup air to a building than isexhausted from the building. An HVAC designer typically provides 10 to 15 percent more outdoor air to abuilding through the air handling units than is exhausted through restroom, kitchen, or laboratory exhaustsystems.

Areas that are positively pressured tend to exfiltrate excess outside air, and areas of negativepressurization have large amounts of infiltration. At best, this infiltration and exfiltration can cause atremendous energy waste to cool and dehumidify the extra outside air. In the case of infiltration, when moistoutside air comes into contact with cool building surfaces inside the wall cavity, condensation and moistureaccumulation will occur. The amount of moisture accumulation can be quite large. For example, theinfiltration of 1 cubic foot per minute (CFM) of outdoor air in Orlando, Florida, can bring in nearly 20 gallonsof unwanted moisture per year. In its worst case, infiltration will create severe humidity problems, mold

growth, and deterioration of building materials.

ASHRAE handbooks provide guidance on mechanical ventilation, and design and control of interior relativehumidity conditions to control microbial growth, to minimize condensation potential, and to provide occupantcomfort, in relation to air leakage.

CONTROL OF HUMIDITY

There are plenty of approaches that can be used to better control the latent energy or moisture insidebuildings.

1) Overcooling with Reheat: Air traveling through the air conditioning equipment can be overcooled toremove additional moisture and then reheated. While some energy codes discourage this approach,there are now some innovative technologies using heat recovery methods to provide the reheat (e.g.heat pipes and rejected heat from cooling cycles).

2) Airflow Reduction: By reducing the flow of air through conventional cooling equipment, latent removalcapability can be increased. However, too much flow reduction can cause freezing in the coil.

3) Enthalpy Heat Recovery: Enthalpy recovery heat exchangers can be used to dry incoming outside airwith dry air being exhausted from buildings.

4) Chiller Control: In large buildings that utilize chillers, cooling systems can operated to vary the SHR inreal time depending on need.

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5) Variable SHR Equipment: Many manufacturers are now making unitary/ package cooling equipmentfor residential and commercial applications that can provide adjustable SHR output.

6) Dedicated Outdoor Air Systems: In many cases buildings are now using dedicated systems thatdehumidify and cool outside air separately from the space conditioning system.

7) Optimized Outdoor Air Control: Outdoor air can be one of the major sources of moisture in a building.CO

2 based ventilation control can regulate the amount of air into a building to provide real time control

of outdoor air ventilation based on occupancy. This ensures that excessive levels of air and humidityare not introduced to the building to add to the moisture removal load.

8) Setback Control Based On Moisture Control: Cooling systems should not be turned off duringunoccupied hours or days. A moderate setback should be used that will control humidity in the space orthe space cooling during setback periods should be controlled based on latent energy concentrations inthe space.

9) Economizer Control Based on Dew Point: Controlling fresh air economizers based on temperature orenthalpy can cause excessive levels of moisture to enter a building. As a result many buildings,including most government buildings are switching to dew point control. The US GSA has establishedthat fresh air economizers should only operate until a 51ºF dew point is reached.

SYSTEM DESIGN ALTERNATIVES A variety of dehumidification systems have been developed to improve the energy efficiency of conventionalreheat systems. Their target is zero reheat and zero overcooling in the dehumidification process by energyrecovery, recycling, reuse and load reduction. These systems include:

1) Conventional reheat systems

2) Run-around coil systems

3) Heat pipe systems

4) Dual-path systems

5) Desiccant systems

CONVENTIONAL COOLING SYSTEMS WITH REHEAT

Install conventional systems in applications with low latent loads, with no requirements for indoor air qualityor humidity control, and where low first cost is a high priority. Consider the use of cooling-coil face andbypass dampers or cold air distribution to reduce the need for reheat.

With conventional dehumidification technology (figure below), warm humid air, flows through a cooling coilwhere it is cooled and dehumidified. If required by indoor space thermostat setting, the dehumidified andcooled air is reheated through a heating coil prior to entering the conditioned space. The cooling coil can bepowered by chilled water from central chiller plant or it can be a direct expansion refrigerant coil. Reheatmay not be needed at peak load conditions but is often needed at low-load conditions with higher latentload fraction.




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Applicability

Conventional reheat systems are most applicable in buildings with:

1) No requirements for humidity control of supply air

2) Dry climates where sensible cooling dominates

3) Low outside air ventilation rate and low space latent load

4) Space relative humidity settings of 55% and higher

5) Low- or no-cost waste heat, steam or hot water available for reheat use

Benefits

Benefits of conventional cooling with reheat include:

1) Simple system configuration

2) Good humidity control by adjusting the off coil air temperature

3) Low initial cost

Design Details

Conventional systems often double the total energy use because of overcooling and reheating of the supplyair. For most climates, there will be long time periods when moisture removal is required but the internalsensible cooling load is significantly lower than the design conditions used to size the air conditioning. Thedehumidifier with variable reheat can be set up to discharge cool air into the space when the room isoccupied, often meeting the internal sensible cooling load without energizing the main air conditioningsystem. Variable reheat, with a narrower range of leaving air temperature, allows the designer to proceedwith more flexibility and can achieve significant operational savings, and often a smaller capacity airconditioning system can be installed.

The need for reheat can be reduced by adopting variable reheat using any of the following approaches:

Dehumidifier with Face and Bypass Damper : Use a bypass damper in parallel with the cooling coil. Theface and bypass damper system regulates the amount of air that flows over the reheat coil and so controls

how much heat is transferred to the air stream. This allows a portion of the air to be cooled to a lowtemperature and dehumidified, and then remixed with the bypass air. The total moisture removal is greaterthan if all the air passes through the cooling coil but is not cooled to as low a temperature. This systemcontrols the discharge temperature to a specific value by regulating the damper positions with a feedbackcontroller.

Dehumidifier with Staged or Stepped Reheat: Stepped or staged reheat will use either a series ofsolenoid valves or an incremental stepped valve to adjust the amount of refrigerant allowed to the reheatcondenser. Adjustments are made in specific increments, such as 20% of refrigerant volume flow. These




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systems provide an intermediate level of control of the reheat condenser. Leaving air temperatures can beheld in a narrower range (±5°F) than with a fixed reheat system, but can not be held at a specific level.

Dehumidifier with Variable Hot Gas Reheat: A variable hot gas reheat system relies on an infinitelycontrolled valve to allow specific amounts of hot gas to the reheat condenser. This system very closelymatches the leaving air temperature to the user’s setpoint. A sensor providing leaving air temperatureinformation to the controller is required. The controller typically uses proportional/integral control logic to

position the hot gas throttling valve.

Use cold air distribution and adjust supply air volume with variable air volume controllers: Anothermeans to reduce reheat requirements is to use variable air volume (VAV) boxes along with variablefrequency drive (VFD) fan motor at the air handler. The supply air is delivered at 50ºF or lower, instead ofthe typical 55ºF and the supply air volume is adjusted instead. Additional benefit is that the lower airflowresults in less fan energy however the downside is that lower chilled water temperatures are necessary andchiller energy consumption may increase. This approach requires careful selection of diffusers to maintaincomfort and is also more susceptible to condensation on ductwork.

Costs

Conventional air handling systems with reheat cost approximately $4.00 to $5.00/cfm. Conventional reheatsystems may have a lower initial cost but their operating costs are much higher because energy is wastedin overcooling and reheating the supply air.

RUN-AROUND COIL SYSTEMS

Install run-around coils in applications with large dehumidification requirements where the air must bereheated after passing the cooling coil.

A run-around coil system is a simple piping loop with an upstream precooling coil and a downstreamreheating coil that sandwiches the main cooling coil. The circulating fluid is pumped to transfer heat fromthe warm mixed air to the off coil cold supply air. The run-around system reduces the cooling load on themain cooling coil; reheat is provided by the heat picked up by the circulating fluid in precooling coil insteadof by an external source of expensive energy.

In new building designs and retrofits, a run-around system can reduce peak heating and cooling loads as

well as total heating and cooling energy. The run-around system can have a significant impact on theheating and cooling capacity in new HVAC designs.

The heat recovery effectiveness of the run-around loop is defined as the ratio of the actual heat transfer tothe maximum possible heat transfer between the air streams. This is equivalent to the ratio of the differencebetween the mixed air temperature and the air temperature off the precool coil to the difference between the




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mixed air temperature and the air temperature off the main cooling coil. The effectiveness ranges from 50%for a normal loop to 65% for a high performance loop. Because of the relatively small temperaturedifferences between the energy exchange coils, low approach cooling coils should be used. Designers mustaccount for the additional pressure drop from the added coil.

Run-around coil systems are most applicable in situations requiring substantial dehumidification.

Benefits

Benefits of run-around coil systems include:

1) Lower cooling load contributes to a smaller cooling system and less pumping energy use, but fanenergy increases due to extra air pressure drop through the run-around coils.

2) Reheat energy is saved

3) Lower total energy use

Design Details

The increased dehumidification capacity provided by runaround coils allows for a smaller cooling system.However, the addition of coils will increase the pressure drop, and fan power must be adjusted accordingly.

The run-around loop can either be applied to existing systems or can be installed at the factory. The run-

around loop requires a fractional horsepower pump, a 120V–60HZ single-phase electrical circuit, and athree-way valve or a variable-speed drive (VSD) for the pump. For bigger systems, an expansion tank withair vent may be needed.

Costs

The initial cost of a run-around system is about double that of a conventional system, but if the downsizingof the chiller and cooling tower is counted, the total initial cost will be very close. The total installation cost isapproximately $4.50 to $5.00/cfm.

The cost effectiveness of a run-around system depends on the system it is replacing. When used instead ofa dehumidifying system requiring reheat, the simple payback is about two to three years. However, whenthe system replaces a system without reheat (no humidity control), there are additional benefits includingincreased comfort and enhanced indoor air quality, which are difficult to quantify.

Operation & MaintenanceRun-around systems require extra maintenance for the two coils and the loop. Air trapped in the coils, pumpand piping must be vented upon initial startup to ensure effective fluid flow and heat transfer. The pre-cooling and reheating function can be controlled by adjusting the pump speed with VSD, cycling the pumpon-off, or using valve control and bypass.

HEAT PIPE SYSTEMS

Install heat pipes in applications with large dehumidification requirements where the air must be reheatedafter passing the cooling coil. Heat pipes increase the effectiveness of air conditioning systems by helpingto decrease the total cooling load of the air. The heat pipe system is hermetically sealed, uses a wickingaction, and requires no pump.

In its simplest form, a heat pipe is a metal tube sealed at both ends, evacuated and charged with avaporizable liquid (refrigerant). The liquid refrigerant at the bottom end readily turns to gas when that end ofthe pipe is warmed, and floats to the top end of the pipe. If that end is in a cooler environment, the gascondenses, releasing heat. The liquid then flows back to the bottom where the cycle begins again. The netresult is heat transfer from bottom to top, without a compressor. Capillary action is sometimes used to helpmove the liquid, allowing for greater flexibility in configuration.

Heat pipes may be described as having two sections: ‘Pre-cool’ and ‘Reheat’. The first section is located inthe incoming air stream. When warm air passes over the heat pipes, the refrigerant vaporizes, carrying heatto the second section of heat pipes, placed downstream. Because some heat has been removed from the




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air before encountering the evaporator coil, the incoming air stream section is called the pre-cool heat pipe. Air passing through the evaporator coil is assisted to a lower temperature, resulting in greater condensateremoval. The "overcooled" air is then reheated to a comfortable temperature by the reheat heat pipesection, using the heat transferred from the pre-cool heat pipe.

Activated by temperature difference and therefore consuming no energy, the heat pipe, due to its pre-cooling effect, allows the evaporator coil to operate at a lower temperature, increasing the moisture removal

capability of the air conditioning system by 50-100%. With lower relative humidity, indoor comfort can beachieved at higher thermostat settings, which results in net energy savings. Generally, for each 1ºF rise inthermostat setting, there is a 7% savings in electricity cost. In addition, the pre-cooling effect of the heatpipe allows the use of a smaller compressor.

Heat pipe performance depends on the temperature difference between the cooling and the reheat coilbanks. As outdoor air temperature decreases, the heat transfer across the heat pipe also is reduced so thateventually little or no reheat is available. Temperatures leaving the air conditioner with heat pipe added mayvary significantly as the heat load changes.

Benefits

Benefits of heat pipe systems include:

1) Removes 50% to 100% more moisture than systems without heat pipes.

2) Saves energy compared to systems that provide similar amounts of dehumidification.

3) Simple system with no moving parts or external connections makes it basically maintenance free.

Design Details

The increased dehumidification capacity provided by heat pipes allows for a smaller cooling system.However, the addition of heat pipes will increase the pressure drop, and fan power must be adjustedaccordingly.

Heat pipes can either be applied to existing HVAC systems or can be installed at the factory. The heat pipeloop is usually controlled by cycling on-off or modulating the refrigerant flow with a control valve.

Costs: The installation cost of a heat pipe loop for a cooling system is approximately $2.50/cfm.

AIR TO AIR HEAT EXCHANGER

Another form of passive energy transfer when using an air conditioning unit for dehumidification of 100%outside air applications is the use of an air to air plate heat exchanger. Heat is transferred from outdoor aircoming into the air conditioner to the cold air leaving it. Again, the goal is to boost the portion of airconditioning capacity used for removal of latent heat by decreasing the need to remove sensible heat. To alimited extent, the exchanger itself will remove latent heat by condensing moisture on the entering air side.




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Outdoor air is introduced into one side of the heat exchanger, and is partially cooled. It then flows over thecold refrigeration coil for moisture removal and additional sensible cooling. The cold, saturated air thenpasses through the other side of the exchanger for warming before being introduced to the air handler orthe space.

The efficiency of the heat exchanger is a function of the size and spacing of the aluminum fins and thetemperature difference. As with the heat pipe, and for the same reasons, the amount of reheat andtherefore the leaving air temperatures can vary significantly.

DUAL-PATH SYSTEMS

Install dual-path systems in applications with return air and large dehumidification load due to high outsideair ventilation rate.

A dual-path system uses two coils (either chilled water or DX) to separately cool the incoming outside airand return air. The hot and humid outdoor air is cooled by a primary coil to 42°F to 45°F fordehumidification. The secondary coil furnishes the sensible cooling of part of the relatively cool and dryreturn air.

A portion of the return air may bypass the secondary coil and mix with the cooled return air stream. Thesetwo air streams are then mixed into supply air with appropriate temperature and humidity.

In systems where the fraction of outside air is low and the space latent load is high, the outside air alonemay not be enough to handle the total latent load of the supply air, which requires some moisture to beextracted from the return air stream. This means a portion of the return air needs to be overcooled to extractmoisture and additional reheat may be necessary to increase the air temperature for comfort supply.

One zero-reheat solution is to direct a portion of the return air to mix with the outside air before

dehumidification.




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In chilled water dual-path systems, the outdoor air (OA) coil can use cold chilled water at 40°F to 42°F forlatent cooling, while the return air (RA) coil can use warmer chilled water at

50°F to 60°F for sensible cooling, thus improving chiller efficiency. Dual-path systems decouple the sensiblecooling and latent cooling of the supply air, thereby improving control of temperature and humidity.

Dual-path systems are best in HVAC applications where the moisture load arises primarily from the outdoorair. These applications include commercial buildings in humid climates, schools, clean rooms, theaters,supermarkets, hotels and motels. For larger systems, separate air-handling units for outside air and returnair can be used.

BenefitsBenefits of dual-path systems include:

1) Reduces the installed cooling tons over a conventional single-path system

2) Provides low operating cost with efficient cooling and no reheat

3) Provides direct control of ventilation air quantity for improved indoor air quality

4) Provides good humidity control at all times, including part load, as moisture is removed at its source,regardless of building load

Design Details

Dual-path systems avoid overcooling and reheating the supply air, thus reducing the size of cooling andheating systems. The sensible cooling of the return air can use chilled water with higher temperature to

improve chiller efficiency. The additional costs of coil, duct and pipe work, and damper or VSD control mustbe adjusted accordingly.

Dual-path systems can be installed separately or integrated with additional HVAC/R equipment. They arecurrently available in factory package units for indoor and outdoor installation. The OA cooling coil shouldbe sized for peak latent load, while the RA cooling coil should be sized for peak sensible load. The OA pathcontrols the humidity of the supply air by modulating the chilled water flow, while the RA path controls thesupply air temperature by adjusting the bypass damper position.




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Costs: The installation price of a dual-path system varies between $5– $6/cfm. Dual-path systems areenergy efficient while assuring an acceptable humidity level at all ventilation air volumes. Its use can alsoreduce demand and energy charges sufficiently to offset the higher first cost.

DESICCANT SYSTEMS

Install desiccant systems in applications requiring large dehumidification and low space humidity levels thatwould be difficult to achieve with cooling-type dehumidification.

Desiccant materials can absorb between 20% and 40% of their dry weight in water vapor from humid air.Both solid and liquid desiccants are used in cooling systems, but solid desiccants are much more commonin commercial buildings. Liquid desiccants employ solutions such as glycol or salts such as lithium chloride(LiCl2).

In solid desiccant systems, desiccant is formed in place in a honeycomb matrix wheel mounted between theprocess air stream and the reactivation air stream; air seals separate the air streams from each other. Thedesiccant wheel rotates slowly (6 to 20 rotations per hour) between the two air streams. The processairflows through the wheel, gives up its moisture to the desiccant and increases dry-bulb temperature (up to120°F), and finally is cooled by coils for comfort supply. After drying the process air, the desiccant wheel issaturated with moisture and rotates slowly into the reactivation air. The hot reactivation air (with

temperature up to 250°F typically required) heats the honeycomb, absorbs moisture released by the hotdesiccant, and is released as exhaust air from the building. Desiccants are also available that can beregenerated at temperatures as low as 120ºF, allowing a greater range of options for heat sources such asheat pumps or solar sources.

Desiccant systems often incorporate heat recovery equipment. If exhaust air is available, it can be used tocool the warm air (leaving the desiccant section) before it passes through the cooling coil. When there is noexhaust air, outside air can be used to cool this warm air, and the heated outside air can be used toreactivate the desiccant.

According to the manufacturers, a well maintained desiccant wheel will last for approximately 100,000 hoursof operation (10 to 15 years) before they need replacement. Costs of desiccant systems are typically given




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in terms of $/CFM. For large commercial systems, the cost is approximately $5/cfm, while smaller units(less than 1000 CFM) may cost up to $8/cfm. Desiccant systems should use low-cost surplus heat, wasteheat or solar heat for desiccant reactivation and where not possible, desiccant systems usually use heatfrom natural gas as their primary energy source. Desiccant systems use very little electric energy and cansave money when the cost of power is high during the peak demand periods of summer. In large buildingswhere non-electric heat is available for reactivation of desiccants, desiccant systems can reduce HVACelectricity use by 30% to 60% and peak electricity demand by 65% to 70%.

The maintenance requirements of desiccant systems can be modest compared to conventional cooling-based dehumidification systems. Filters located in the inlet of process air and reactivation air needs to becleaned or replaced every two months. About 90% of reported problems related to desiccant systems canbe traced to clogged filters. The wheel can be vacuumed to remove dust from the wheel face. The drive beltaround the heat wheel needs to be tight enough to turn the wheel without putting excessive load on thedrive motor shaft bearings. No regular maintenance is required for the desiccant materials.

ENERGY RECOVERY ENTHALPY WHEEL

Energy recovery wheel can be incorporated into a ventilation system to transfer both the sensible and latentenergy between outdoor and exhaust air streams. The core of an enthalpy wheel can be made from avariety of materials such as paper, metal or plastic, which is coated with a desiccant. This coating enables

the wheel to transfer both sensible and latent energy between air streams.

The enthalpy wheel provides first stage cooling or heating. For instance, in the summer the wheel removesmoisture from incoming air and pre-cools it to lighten the load on the evaporator coil. Likewise, in coldertemperatures (winter application) the wheel will inject moisture into supply air and pre-warm it to reduce theload on the reheat coil. In the summer and winter modes, the wheel saves energy.

There are three general types of wheels being used today. They are sensible, enthalpy and regenerative.

Sensible wheel - This wheel is not coated with a desiccant and therefore transfers only sensible energy.The wheel can be constructed of almost any material (paper, metal or plastic) and transfers energy betweentwo air streams as the mass of the material gains or loses heat to the opposite air stream. The wheelrotates at a speed of 25 to 50 revolutions per minute.

Enthalpy wheel - It is similar to the sensible wheel except that a desiccant media is added to the wheel’s

surface. As the wheel rotates, it now can transfer sensible energy and humidity. This wheel also rotates at25 to 50 revolutions per minute.

Regeneration wheel- This wheel is used when low dewpoint conditions (<45°F) are required, such asindustrial applications. It achieves low dewpoints by slowing the wheel to a speed of between 0.25 and 1revolution per minute and by using an air stream heated to 250°F or more to drive off moisture andregenerate the wheel.

This heated air stream is typically focused on only 1/4 of the wheel’s area thereby allowing 3/4 of the areato be available for the process side.

ENERGY RECOVERY V/S DESICCANT WHEELS

Desiccant wheels are often confused with energy recovery wheels. The confusion is understandable. Bothdevices look nearly the same, and both types are constructed with honeycomb media and containdesiccant. Also, sensible- only heat wheels are sometimes used as post-coolers in desiccant systems. Butthere are important functional differences between these devices which appear so similar.

Heat wheels are optimized to transfer sensible heat between two air streams, while desiccant wheels areoptimized to remove moisture. These different purposes lead to differences in materials and in wheelrotation speed. An energy recovery wheel rotates at a comparatively high speed (20 rpm), to maximize theheat transfer between air streams. A desiccant wheel rotates 60 times more slowly (10 to 20 rotations perhour). The slow rotation speed allows the desiccant to adsorb more moisture, and it minimizes the amountof heat carried over from the hot reactivation air into the cooler process air.




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If the exhaust air is dry, an energy recovery wheel can transfer some moisture out of the incoming air. Butenergy recovery wheels contain less desiccant than desiccant wheels. Also, the honeycomb material, airseals and support structure of an enthalpy wheel cannot endure the temperature and moisture differencestypical of desiccant wheel operation.

Consequently, the wheels perform quite differently.

Desiccant wheels compared to energy recovery wheels

As seen in figure above, thermal energy used for reactivation allows desiccant wheels to remove muchmore moisture than energy recovery wheels. Desiccant wheels can deliver air below the moisture conditionof the exhaust air. That level of dryness cannot be reached with energy recovery wheels.

Conclusions:

Some important observations and field results:

1) A standard cooling system does not provide enough dehumidification at low load when the supply airvolume remains constant. Standard cooling with reheat provides good humidity control at the cost ofhigher energy consumption, which is discouraged by model energy codes. Variable air volume systemsinstead provide a suitable alternative to this problem.

2) Run-around coil systems and dual-path systems are energy efficient in dehumidification applications.They reduce coil loads and avoid reheat for most load conditions. The dual path system design uses aseparate cooling coil for outside air.

3) Run-around systems and dual-path systems may need reheat at very low load conditions when spacelatent load dominates the total cooling load and the ventilation rate remains constant. A special caseoccurs in supermarkets where refrigerated display cases cool the store during unoccupied hours or

when the store is cool due to cool weather. In this case, dual-path systems are equipped with reversingvalves that allow the return air circuit to provide heating.

4) Pretreatment of outdoor air for moisture removal can provide a cost-effective way to prevent moisturefrom creating indoor air quality problems. When pretreatment is used, various approaches for energyrecovery are available when recovery from exhaust air is not a possibility. Heat pipes, air-to-air heatexchangers, and some dehumidifiers recover energy via fixed reheat. While these methods can providesufficient moisture removal capacity and do recover at least some energy, the level of temperaturecontrol is rather limited. The air handler must usually heat or cool to adjust the temperature.




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5) Dehumidifiers with variable reheat both remove moisture and regulate the amount of hot gas reheat tocontrol leaving air temperature. The annual energy savings over fixed reheat are significant, becausethe need for further heating and cooling is much decreased. The most precise control, and therefore thegreatest energy savings, is provided by a dehumidifier with variable hot gas reheat.

To achieve an energy-efficient dehumidification system design, consider the following factors:

1) Size and select cooling coils with enough cooling capacity to handle the peak sensible cooling load andpeak latent cooling load that occur at different load conditions. Use low approach cooling coils and lowtemperature water.

2) Design systems considering various load conditions rather than only the peak load condition. Forconventional cooling with reheat systems, size reheat equipment to handle a higher reheat requirementin lower load conditions, especially for constant-volume systems.

3) Integrate heat recovery equipment into conventional cooling systems to reduce cooling loads andreheat energy. Runaround loop systems are much more energy efficient than conventional coolingsystems, especially when operating in part-load conditions.

4) Dual-path systems offer competitive energy efficiency with run-around loop systems, and provide bettercontrol of the outside air ventilation rate. Dual-path systems decouple sensible cooling and latentcooling for easy control of the supply air temperature and humidity. Equipment is available to provide

both cooling and reheat.5) Desiccant systems are more competitive when a low supply air dew-point temperature is required,

latent load fraction is high, low- or no-cost reactivation heat from steam, hot water or waste heat isavailable, and electricity costs are high when compared to gas costs.

6) Lay out equipment correctly. Place filters upstream of coils. Place fans downstream of coils (draw-through mode) to provide a small amount of reheat.

7) Select low face velocity coils to reduce air pressure drop and improve dehumidification performance. .Where humidity loads are high, consider deeper (high row) evaporator coils.

SECTION-8 CONSTRUCTION OF CONTROLLED SPACE

As much as 90 percent of all water intrusion problems occur within one percent of the total building exteriorsurface area. The one percent of the structure’s façade contains the terminations and transition detailingthat all too frequently lead to envelope failures. Construction detailing is a critical component for thesuccess of any wall assembly. The designer must create details that effectively:

1) control rain water penetration that may occur via gravity flow, capillary action and pressure differentialdue to the effects of wind pressure, stack effect or mechanical ventilation that create pressuredifferences across the building envelope, and drive water through cracks or openings

2) control condensation that may occur via air leakage and diffusion

It is very important to take certain precautions in preparing space for the humidity control, regardless of the

type of air drying equipment or the method used to do the drying. Satisfactory moisture control-better knownas customer satisfaction-depends on many variables.

The Nature of Water Vapor

Consider two closed rooms, adjacent to one another. If the partial pressure of the water vapor in room #1 isgreater than the partial pressure of the water vapor in room #2, then the water vapor will travel through thewall into room #2 regardless of the composition of the wall.




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Let's take the hypothetical example a step further. If the absolute humidity of the air in room #1 is greaterthan that of the air in room #2, then the water vapor pressure will be higher in room #1. Therefore, whendrying room #2, the problem of new water coming through the wall from room #1 must be considered.

A vapor barrier can slow down the passage of vapor from wet to drier areas, but it cannot keep water out; itcan only slow the rate of penetration. Commercial vapor barriers-moisture resistant construction materialincluding paints and other coatings offer a variety of design alternatives. The choice of vapor barrier is

based on the degree of dryness required in the controlled space, the efficiency of the equipment being usedfor drying, and the cost of construction. When preparing space for the humidity control (permeation of watervapor), the following considerations are important to effectively control condensation and prevent moisturepenetration:

Design Considerations

• Air permeability

• Continuity with other air barrier materials

• Structural integrity

• Durability

• Water penetration resistance

• Water vapor permeability

• Code compliance

• Climate

Air Permeability: The layers of material that make up a wall assembly have different air permeability.Energy codes in the United States have begun to require air tightness of the building envelope, but they arenot specific about levels of air permeability for air barrier materials. The generally accepted level based onNational Building Code of Canada requirements is 0.004 cfm/ft

2 at 1.57psf. While many common building

materials like plywood and gypsum wallboard meet this standard, a sheathed wall assembly will not performwell as an air barrier unless the joints are treated with an air barrier material.

Air Barrier Continuity: Any vapor barrier must be continuous, without breaks or tears. In cold climates thebreaks can allow significant amounts of warm moisture-laden air to escape from the interior environmentand condense on a cold surface in the wall assembly. Conversely, in hot, humid climates, breaks in the airbarrier permit moisture-laden air from the exterior environment to infiltrate the building envelope andpotentially condense on a cold surface in the wall assembly.

Air Barrier Structural Integrity: Structural integrity of air barriers is important because if the materials tearor displace with wind loading, they lose their effectiveness as air barriers. Some building wraps have low airpermeability, but they do not perform well when commonly installed because they have many seams. All lap joining must be tightly closed (this is particularly critical when mechanical or caulked joints are used).

Air Barrier Durability: While capable of resisting wind loads without compromise in performance, airbarrier materials must also demonstrate durability in a number of other ways, particularly if the air barrier isconcealed and inaccessible for maintenance. Durability criteria include:

• Resistance to puncture

• Resistance to pests—rodents, termites, carpenter ants, and other insects

• Resistance to low but sustained negative pressures from building stack effect and HVAC fan effect




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• Ability to withstand stress from thermal and moisture movement of building materials, and stressfrom building creep

• Resistance to UV degradation (during the construction period)

• Resistance to mold growth

• Resistance to abrasion

Water Penetration Resistance: The traditional moisture protection used in wall construction is asphalt-saturated felt or kraft waterproof building paper. Building wraps are often used in place of asphalt felt in wallconstruction, often with the same perceived purpose. Seamless fluid applied moisture protection provides asignificant improvement over traditional moisture protection and building wraps. In fact, these can be 10times more resistant to water penetration than building wraps and nearly 200 times more resistant to airleakage than asphalt felt.

Water Vapor Permeability: The generally accepted definition of a vapor-retarding material is one that hasa water vapor permeance of 1.0 perms or less. Table below provide Water vapor permeance of commonmoisture barrier and building materials.

Building Material Water Vapor Permeance(Perm)

4 mil Polyethylene 0.08

6 mm (1/4 inch) Plywood3 (extglue)

0.7

101mm (4 inch) Brick3 0.8

203mm (8 inch) Concrete Block3 2.4

25mm (1 inch) ExpandedPolystyrene1

5

Type 15 Building Felt2 5.6

Fluid Applied Air MoistureBarrier Waterproof Coating

5.7

19mm (3/4 inch) Plaster onMetal Lath3

15

Fluid Applied Air MoistureBarrier Joint Treatment

17.3

9.5mm (3/8 inch) GypsumWallboard3

50

The purpose of a vapor retarder in wall construction is to minimize water vapor diffusion through the wallassembly and thus reduce the risk and the amount of condensation on cold surfaces in the wall assembly.




Whether or not a vapor retarder should be placed in a wall assembly and where it should be placed must becarefully evaluated in relation to climate, the physical characteristics of other components of the wallassembly, and interior relative humidity conditions.

1) In cold climates the predominant water vapor diffusion direction through most of the year is from theinside to the outside, as warm, humid air from the interior environment moves in the direction of cold,dry outside air.

2) Conversely, in hot, humid climates, the predominant water vapor diffusion direction through most of theyear is from the warm, humid outside environment towards the cooler, dryer, air-conditioned interiorenvironment.

Based on these general conditions, a vapor retarder is customarily placed on the interior of wallconstruction in cold climates and on the exterior in hot, humid climates. A vapor retarder should not beplaced on the interior in hot, humid climates, since it will potentially cause condensation by restricting vapordiffusion to the interior. The use of interior vapor retarders has been shown to be a contributing cause inmany cases of moisture problems and IAQ problems in buildings in hot, humid climates. Refer ASHRAEHandbook—Fundamentals, chapters 21 and 22) for further information.

Insulation between vapor barriers can be a potential problem: if construction occurs in humid weather, watercan be "sealed in" between the two vapor barriers. Sealed-in vapor will travel into the controlled space andimpose an extra drying load on the drying equipment. This extra load lasts only until the insulation dries out,

but meanwhile, humidity control is difficult.

Code Compliance

Model building codes and state and municipal codes in the United States do not address air barriers,moisture barriers and vapor retarders in a uniform way.

1) Energy codes in the United States, including the IECC (International Energy Conservation Code), theState of Massachusetts Building Code, and ASHRAE 1999 energy conservation standard(ANSI/ASHRAE/IESNA Standard 90.1-1999, Energy Standard for Buildings Except Low-RiseResidential Buildings, an energy conservation standard which is required to be adopted by statebuilding energy codes under the Federal Energy Conservation and Production Act) require airtightening of the building envelope. Although codes in the United States do not always provide specific

limits for air leakage of air barrier materials, the generally accepted limit is 0.004 cfm/ft 2 at 1.57psfpressure [0.02 L/(s·m

2) at 75 Pa pressure] based on National Building Code of Canada requirements.

2) Most model codes generally require the use of a water-resistive barrier in wall construction andprescribe asphalt saturated felt (IBC Chapter 14, paragraph 1404.2). They often require the use ofvapor retarders in wall construction (IBC Chapter 14, paragraph 1403.3) unless other means areprovided to avoid condensation.

3) Southern Building Code Congress Public Safety Testing and Evaluation Services, Inc. publishes anEvaluation Guide on Floor, Wall, and Roof Systems (Testing for Moisture Protection Barriers—SBCCIPST & ESI Evaluation Guide 119), which lists specific performance criteria for air and moisture barriers,including fluid applied air/moisture barriers.

By constructing an airtight building envelope, the risk of moisture problems—decay, corrosion, loss ofinsulation value, mold growth and indoor air quality (IAQ) problems—which can occur because of airleakage and condensation, are minimized.

dehumidification in industrial and building applications.pdf

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